1. Field of the Invention
This invention relates generally to the field of offset lithographic printing presses and more particularly to a lockup apparatus for holding a printing plate on the surface of a printing plate cylinder.
2. Description of the Related Art Including Information Disclosed under 37 C.F.R. Section 1.97-1.99
In offset lithographic printing presses a thin flexible printing plate carrying a planographic image is releasibly held on the surface of a plate cylinder. To achieve optimal printing quality it is crucial that the printing plate conform to the surface of the plate cylinder. Securement of the printing plate to known plate cylinders is performed by providing a longitudinal mounting slot in the plate cylinder and inserting an edge of the printing plate therein. The printing plate edge is releasibly held within the slot by a lockup holding device.
In U.S. Pat. No. 5,010,818 to Wallschlaeger, a tensionless plate lockup apparatus having a locking element with a curved surface contacts the ends of a printing plate thereby clamping the ends against a slot surface in the printing plate cylinder. A plurality of locking elements are disposed at predetermined intervals along the length of the slot in the printing plate. A spacer bar having a plurality of upwardly extending mounting posts is placed within the slot to maintain each locking element in a fixed position. The lockup apparatus is configured to secure the edges of the printing plate against a surface of the slot for rotation of the plate cylinder in either a clockwise or counterclockwise direction. However, to accomplish this, the slot width must be very narrow to facilitate the clamping member to contact the clamping surfaces on both sides of the slot. In order to contact both biased surfaces simultaneously the force of the spring and the centrifugal force acting on the clamping member must be shared equally between the two biased surfaces such that the force acting to clamp the printing plate is reduced by the amount of force wasted on the unused biased surface.
Disadvantageously, in this embodiment the fixed securement of the locking elements onto the mounting posts of the spacer bar does not permit pivotal movement of the mounting posts. Furthermore, the mounting arrangement of the locking elements on the spacer bar creates lateral translational movement of the mounting posts upon insertion to the slot of the plate cylinder. Additionally, the rigid engagement of the stationary locking element establishes lateral frictional forces against the slot surface thereby creating an uneven wear of the locking element.
In an alternative embodiment shown in U.S. Pat. No. 5,010,818, the printing plate is tightly secured to the cylinder body in order to reduce plate cracking. However, in this embodiment dedicated cylinder rotation is required in either a clockwise or counterclockwise direction. Therefore, this alternative design cannot be universally used in a press configuration where select cylinders are required to operate in a reverse direction for different print layouts.
In U.S. Pat. No. 4,493,258 to Wallschlaeger et al. a plate lockup mechanism has an elongated cam with an arcuate surface and a generally V-shaped removed portion which rotates on the bottom surface of a plate roll groove. A bar-like toggle having arcuate lower surfaces is mounted on the arcuate outer surface of the cam. A securing member carried on the toggle engages the edge of a printing plate fixedly holding the plate edge. The leading edge of the plate is held against the clamping surface of the slot by a fixed spring. The trailing edge of the plate is held in tension by a toggle mounted spring in contact with the plate edge. The toggle is positioned by activating the cam in which the radius of the cam interfaces with a radius on the lower portion of the toggle causing the toggle to rotate and tension the spring on the plate edge.
Disadvantageously, the trailing edge of the plate is positioned by the spring making the location lockup dependent, thus allowing for a poor fit between the plate and the cylinder body. Additionally, when the plate is held by tension, the plate is more susceptible to circumferential slippage resulting in poor registration and plate cracking. Friction between the rigid securing members and the plate edge also creates lateral tangential sheering forces upon the high speed rotation of the cylinder.